Method for heat treating with a gradual temperature profile

ABSTRACT

A method for heat treating a crankshaft for a vehicle propulsion system, or other work piece, includes heating at least a portion of the crankshaft to a gradual temperature profile. The temperature profile has gradually lower temperatures from the surface to the core of the crankshaft. The temperature profile includes a midpoint temperature at a midpoint between the surface and an innermost part of the core, the midpoint temperature being at least 50% of the surface temperature as measured in the Celsius scale. The surface temperature is within a transformation range of the crankshaft material. The method further includes quenching the surface of the crankshaft journal. The material of the crankshaft is preferably a carbon steel alloy having a DI less than 1.7 and having greater than 0.3 wt % carbon.

FIELD

The present disclosure relates to a system and method for heat treating a crankshaft for a vehicle propulsion system.

INTRODUCTION

An engine's crankshaft converts reciprocating linear movement of a piston into rotational movement about a longitudinal axis to provide torque to propel a vehicle, such as, but not limited to, a train, a boat, a plane, or an automobile. Crankshafts are a vital part of an engine and are a starting point of engine design.

The crankshaft includes at least one crankshaft pin bearing journal that is offset from the longitudinal axis, to which a reciprocating piston is attached via a connecting rod. Force applied from the piston to the crankshaft through the offset connection therebetween generates torque in the crankshaft, which rotates the crankshaft about the longitudinal axis, which is the rotational axis. The crankshaft further includes at least one main bearing journal disposed concentrically about the longitudinal axis. The crankshaft is secured to an engine block at the main bearing journals. A bearing is disposed about the main bearing journal, between the crankshaft and the engine block.

The crankshaft pin and main bearing journal surfaces are typically hardened to be able to handle the load and wear. One approach for hardening is to inductively heat and then quench to harden the crankshaft journal surfaces. With induction heating/hardening, high-frequency alternating current is used to induce an eddy current in the surface zone of the work piece that is to be hardened. These eddy currents result in Joule heating which causes rapid heating of the work piece to a certain temperature. Hardening is then accomplished by rapidly quenching.

The induction hardening of crankshafts has created problems in the past. One problem is that, while induction hardening increases hardness and strength, the volumetric growth accompanying the phase change from hardening generates residual stresses. When the components of these residual stresses combine with working stresses, they impose a detrimental risk of promoting premature fatigue failure initiated in the subsurface between the hardened surface layer and the unhardened core. The residual stresses are a result of temperature variation in heating and cooling of the object and the volumetric change in hardening due to the specific volume difference between the original and new formed phases in the steel. If that sub-surface material is substantially stressed, the material may develop cracks that may propagate and result in failure of the crankshaft.

Conventional attempts to alleviate or reduce the residual tensile stresses caused by induction hardening have included pre-heating and/or post-hardening (e.g., high temperature) tempering the entire crankshaft in an oven or furnace. However, these conventional methods have a number of challenges, including cost, time, and marginally effective results.

SUMMARY

The present disclosure provides a method of hardening that utilizes induction heating to generate within the work piece a gradual temperature profile prior to quenching. This gradual profile results in more evenly distributed tensile stresses throughout the workpiece, rather than concentrating the tensile stresses near the subsurface between the hardened surface and unhardened core.

In one form, which may be combined or separate from other forms disclosed herein, a method for heat treating a crankshaft for a vehicle propulsion system is provided. The crankshaft is preferably formed of a crankshaft steel alloy. The method includes heating at least a portion of the crankshaft to a temperature profile having a surface temperature at a surface of the crankshaft. The temperature profile of the crankshaft has gradually lower temperatures from the surface to a core of the crankshaft. The temperature profile includes a midpoint temperature at a midpoint between the surface and an innermost part of the core. The midpoint temperature is at least 50% of the surface temperature, as measured in the Celsius scale. The surface temperature is within a transformation range of the crankshaft steel alloy. The method further includes quenching the surface of the crankshaft.

In another form, which may be combined with or separate from the other forms disclosed herein, a method for forming a crankshaft for a vehicle propulsion system is provided. The method includes forming the crankshaft, preferably, of a crankshaft steel alloy and forming a round bearing journal surface about the crankshaft. The method further includes applying a series of inductive heating pulses to the crankshaft until the crankshaft has a gradual temperature profile extending normally from the round bearing journal surface to an innermost part of the core of the crankshaft. The gradual temperature profile includes a surface temperature at the round bearing journal surface, the surface temperature being within a transformation range of the crankshaft steel alloy. The gradual temperature profile also includes a midpoint temperature at a midpoint between the round bearing journal surface and the innermost part of the core. The midpoint temperature is at least 50% of the surface temperature. The method includes quenching the round bearing journal surface to a temperature below the transformation temperature to harden the round bearing surface.

In yet another form, which may be combined with or separate from the other forms disclosed herein, a method of induction hardening a work piece is provided. The method includes providing a work piece formed of a work piece material and having an outer surface. The method further includes applying a series of inductive heating pulses to the work piece until the workpiece has a gradual temperature profile extending normally from the outer surface to an inner part of the work piece. The gradual temperature profile includes a surface temperature at the outer surface, where the surface temperature is within a transformation range of the work piece material. The gradual temperature profile includes a midpoint temperature at a midpoint between the outer surface and the inner part. The midpoint temperature is at least 50% of the surface temperature. The method further includes quenching the outer surface to a temperature below the transformation temperature to harden the outer surface.

Additional features may be provided, including but not limited to the following: wherein heating the portion of the crankshaft includes induction heating the portion of the crankshaft; wherein the temperature profile includes a 25^(th) percentile temperature at a 25^(th) percentile point that is halfway between the midpoint and the surface, the 25^(th) percentile temperature being within 10% of the surface temperature; wherein the temperature profile includes a 75^(th) percentile temperature at a 75^(th) percentile point that is halfway between the midpoint and the innermost part of the core, the 75^(th) percentile temperature being at least 50% of the surface temperature; the midpoint temperature being at least 70% of the surface temperature; wherein the midpoint temperature is in the range of 70% to 80% of the surface temperature; wherein the 75^(th) percentile temperature is in the range of 60% to 70% of the surface temperature; wherein the crankshaft surface is located on a round bearing journal surface of the crankshaft and the temperature profile extends along a radius of the bearing journal surface to the innermost part of the core of the crankshaft; providing the crankshaft material as a steel having an ideal critical diameter (DI) less than 1.70; the steel being a carbon steel having at least 0.3 wt % carbon; wherein heating the portion of the crankshaft includes applying a plurality of inductive field pulses to the crankshaft; wherein heating the crankshaft includes applying a first inductive pulse with an intensity in the range of 2.0 to 2.5 J/C*mm², applying a second inductive pulse with an intensity in the range of 2.0 to 2.5 J/C*mm², applying a third inductive pulse with an intensity in the range of 2.0 to 2.5 J/C*mm², and pausing between the application of each of the first, second, and third inductive pulses; applying the first inductive pulse for a first period; applying the second inductive pulse for a second period; applying the third inductive pulse for a third period; each of the first, second, and third periods being in the range of 8 to 12 seconds; wherein inductively heating the crankshaft comprises applying an alternating current to a coiled conductor; and wherein pausing includes pausing for a pause period between 1 and 3 seconds between the application of each of the first, second, and third inductive pulses.

Further areas of applicability of the present disclosure will become apparent from the detailed description provided below. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

FIG. 1 is a fragmentary side elevation of a portion of a crankshaft with an induction heating coil disposed around an outer bearing journal surface, in accordance with the principles of the present disclosure;

FIG. 2 is a graph illustrating an exemplary temperature profile used in a method of induction hardening a crankshaft journals, in accordance with the principles of the present disclosure;

FIG. 3 is a graph illustrating another exemplary temperature profile used in a method of induction hardening crankshaft journals, in accordance with the principles of the present disclosure; and

FIG. 4 is a graph illustrating martensite formation as a function of depth during an induction hardening method for crankshaft journals, according to the principles of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to several examples of the disclosure that are illustrated in accompanying drawings. Whenever possible, the same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form and are not to precise scale. For purposes of convenience and clarity only, directional terms such as top, bottom, left, right, up, over, above, below, beneath, rear, front, inner, and outer may be used with respect to the drawings. These and similar directional terms are not to be construed to limit the scope of the disclosure in any manner.

Referring now to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures, FIG. 1 is a fragmentary side elevation of a portion of a crankshaft 100 with an induction heating coil 102 disposed around an outer surface 104 of a pin journal 108 of the crankshaft 100. In this example, the outer surface 104 of the pin journal 108 of the crankshaft 100 is a round bearing journal surface of the crankshaft 100. The crankshaft 100 includes a pair of counterweights 106 connected via the pin journal 108. The outer surface 104 of the pin journal 108 transitions into the counterweights 106 via rounded corners or fillets 110. While the exemplary crankshaft 100 of FIG. 1 includes on a single pin journal 108, it is to be appreciated that a crankshaft may include any number of additional pin bearing journals, main bearing journals, and counterweights as required for the particular engine for which it is designed, without limitation.

In accordance with an exemplary aspect of the present disclosure, the outer surface 104 of the pin journal 108 is heated by the induction heating coil 102 to ultimately harden the surface 104. The induction heating coil 102 may be of any desired configuration. The induction heating coil 102 may be energized from a suitable source of high frequency alternating electric current, which causes a high density alternating current to be induced to flow through the crankshaft 100 in the pin journal 108, which, in turn, generates heat within the pin journal 108.

The present disclosure provides a method for heat treating the surface 104 of the pin journal 108, which could be applied to any surface of the crankshaft 100, to reduce stresses that may otherwise be caused by induction hardening. In an exemplary aspect of the present disclosure, induction heating is performed on the crankshaft 100 to provide a temperature profile extending inwardly from the surface 104 that gradually lessens.

Referring to FIG. 2, a graph 200 illustrates an exemplary induction heating profile 202 in accordance with the present disclosure. The horizontal axis 204 of the graph 200 corresponds to the depth in millimeters from the surface 104, while the vertical axis 206 corresponds to material temperature in degrees Celsius. In the example illustrated in graph 200, the surface temperature S of the surface 104 reaches about 920 degrees Celsius, which is in a transformation range of the steel alloy of the crankshaft 100. In other words, the steel alloy at the surface 104 becomes austenitized when heated to the surface temperature S.

The temperature profile 202 includes gradually lower temperatures from the outer crankshaft surface 104, at 0 mm along the axis 204, to an innermost part 112 of the core of the crankshaft 100. The temperature profile 202 extends normally along a radius of the bearing journal surface 104 to the innermost part 112 of the pin journal 108 of the crankshaft 100.

In this example, the crankshaft 100 is solid and the innermost part 112 of the core of the crankshaft 100 is located along the longitudinal axis L (which is also the rotational axis of the crankshaft 100). In this case, the innermost part 112 of the solid core is located at 25 mm from the surface 104 along the longitudinal axis L. In other examples, the crankshaft 100 could be hollow, and in such case, the innermost part 112 of the core could lie on an internal surface of the crankshaft 100 that is offset from the rotational axis thereof.

The temperature profile 202 includes a midpoint temperature M at a midpoint 208 between the outer crankshaft surface 104 and the innermost part 112 of the core. The temperature profile 202 has gradually lower temperatures going from the surface 104 to the innermost part 112 of the core. As the changes in the profile 202 are gradual along the profile 202, the midpoint temperature M is at least 50% of the surface temperature S.

As used herein, the percentages of temperatures are measured with respect to the Celsius scale. Accordingly, for example, the midpoint temperature M is at least 50% of the surface temperature S, as measured using the Celsius temperature scale.

Referring to FIG. 2, data points corresponding to points on the temperature profile curve 202 are shown in the following Table 1.

TABLE 1 Temperature profile 202 as a function of distance from the surface 104. Distance from Temperature Percentile from Surface 104 (mm) (deg. Celsius) Surface 104 0 920 0 0.25 920 1 0.5 920 2 0.75 920 3 1 920 4 1.25 919 5 1.5 918 6 1.75 917 7 2 916 8 2.25 914 9 2.5 912 10 2.75 910 11 3 908 12 3.2143 905 12.9 3.4286 903 13.7 3.6429 901 14.6 3.8571 898 15.4 4.0714 895 16.3 4.2857 892 17.1 4.5 889 18 4.7143 886 18.9 4.9286 882 19.7 5.1429 879 20.6 5.3571 875 21.4 5.5714 871 22.3 5.7857 867 23.1 6 863 24 6.25 858 25 6.4975 853 26 7.4986 831 30 8.4987 808 34 11.5406 732 46.2 12.5 712 50 17.8031 605 71.2 18.75 593 75 21.4419 558 85.8 24.54534 530 98.2 25 530 100 Table 1 shows data points for the temperature profile 202 as a function of distance from the surface 104 in millimeters. The third column further shows the percentile of depth along which each temperature and depth data point falls. Thus, for example, at 0.25 mm from the surface, the temperature profile 202 has a temperature of 920 degrees Celsius, and this is 1 percentile away from the surface 104 toward the innermost part 112 of the core (which is along the longitudinal axis L of the crankshaft 100, in this example).

As discussed above, the midpoint temperature M is at least 50% of the surface temperature S. The midpoint 208 is located at the 50^(th) percentile of the depth, or halfway between the surface 104 and the innermost part 112 of the core. In this particular example, the midpoint temperature M is 712 degrees Celsius, while the surface temperature S is 920 degrees Celsius. Therefore, in this example, the midpoint temperature M is greater than 70% of the surface temperature S; and more particularly, the midpoint temperature is about 77% of the surface temperature S. It should be understood, however, that the temperature profile 202 may have some variation without falling beyond the spirit and scope of the present disclosure. For example, in some cases, the midpoint temperature M may be in the range of 70% to 80% of the surface temperature S. Similarly, the other temperatures in Table 1 may vary, for example, by up to 10%, or even by up to 30% in some cases. For example, different materials could be used for the crankshaft 100 or other work piece, which would cause the temperature profile to vary from the exact temperature profile 202 shown in FIG. 2 and in Table 1.

As can be seen from the graph 200 and from Table 1, the temperature profile 202 includes a 25^(th) percentile temperature T₂₅ at a 25^(th) percentile point 210 that is halfway between the midpoint 208 and the outer surface 104. In this case, the 25^(th) percentile temperature T₂₅ is within 10% of the surface temperature S. More particularly in this case, the 25^(th) percentile temperature T₂₅ is 858 degrees Celsius. The surface temperature S is 920 degrees Celsius, and therefore, the 25^(th) percentile temperature T₂₅ is greater than 93% of the surface temperature S, but less than 94%.

Furthermore, as can be seen from the graph 200 and from Table 1, the temperature profile 202 includes a 75^(th) percentile temperature T₇₅ at a 75^(th) percentile point 212 that is halfway between the midpoint 208 and the innermost part 112 of the core. In this case, the 75^(th) percentile temperature T₇₅ at least 50% of the surface temperature S. More particularly, in this case, the 75^(th) percentile temperature T₇₅ is 593 degrees Celsius. The surface temperature S is 920 degrees Celsius, and therefore, the 75^(th) percentile temperature T₇₅ is between 60% and 70% of the surface temperature S.

Certain materials are more amenable than others for providing the gradual temperature profile 202 that reduces stresses. In this example, the crankshaft 100 is preferably formed of a steel, such as a carbon steel having greater than 0.3 weight percent carbon. The steel may have an ideal critical diameter (DI) less than 1.70. In other variations, the steel may be provided having an ideal critical diameter (DI) less than 3.0. Some examples of materials that may be used for the crankshaft 100 include 1541 steel, 1545 steel, 1440 steel, 1040 steel, as well as microalloys, such as 1538 MV or 44MnSiVS6 steel.

During the induction hardening process, the surface temperature S is raised to a temperature that is at or above an AC3 temperature of the crankshaft pin journal surface material. An AC3 temperature may correspond to a temperature at which transformation of ferrite to austenite is completed during heating. The temperature profile 202 has gradually lower temperatures along the profile 202, and at the innermost part 112 of the core, the temperature profile 202 has a temperature that is below the austenitizing temperature. However, more than just the surface 104 and the portion of the crankshaft 100 immediately below the surface 104 is heated to a temperature above the austenitizing temperature. As a result, residual tensile stresses developed within the crankshaft 100, rather than presenting only at the surface 104 and abutting compressive stresses near the surface. Thus, the method of the present disclosure provides for deep heating of the crankshaft 100 during the induction hardening process itself. The resultant surface 104 may have a hardness of at least 50 HRC.

In one exemplary inductive heating method, the inductive heating process includes applying a series of inductive heating field pulses via the coil 102 into the crankpin pin journal 108 of the crankshaft 100. For example, the inductive heating method may include applying a first inductive pulse for a first period, pausing, applying a second inductive pulse for a second period, pausing, and applying a third inductive pulse for a third period. In one example, each inductive heating pulse is applied at an intensity of 2.0 to 2.5 J/C*mm², such as 2.25 J/C*mm². Each of the first, second, and third periods last for a duration in the range of 8 to 12 seconds, such as about 10 seconds, so that the application of the field pulse is applied continuously for each of the first, second, and third periods before pausing. The pauses between each period of inductive field application may be in the range of 1 to 3 seconds, or about 2 seconds, by way example. In an exemplary aspect, the pulsing may be achieved by periodically turning an alternating current in a conductive coil in a tool on and off and/or cycling the intensity of the inductive field that is generated by the induction coil.

Pulsing of the inductive field applications allows the crankshaft material to be deep heated with the gradual temperature profile 202. However, it should be understood that the temperature profile 202 may be created in any suitable way, such as by a single application of a high intensity inductive field.

After the crankpin crankshaft pin journal 108 is heated to a gradual temperature profile 202, the method herein includes quenching the outer surface 104 to a temperature substantially below the transformation temperature to harden the outer surface 104. The quenching is shallow and rapid to quickly cool down the outer surface 104. For example, a polymer quench in a water solution may be applied to the outer surface 104.

Referring now to FIG. 3, a graph 300 illustrates an alternative exemplary induction heating profile 302, in accordance with the present disclosure. The induction heating profile 302 may be used with the method described above in place of the profile 202. The induction heating profile 302 may be accomplished with similar inductive heating pulses as described above, by way of example. The horizontal axis 304 of the graph 300 corresponds to the depth in millimeters from the surface 104, while the vertical axis 306 corresponds to material temperature in degrees Celsius. In the example illustrated in graph 300, the surface temperature S′ reaches about 850 degrees Celsius, which is in a transformation range of the steel alloy of the crankshaft 100. In other words, the material at the surface 104 becomes austenitized when heated to the surface temperature S′.

The temperature profile 302 includes a midpoint temperature M′ at a midpoint 308 between the outer crankshaft surface 104 and the innermost part 112 of the core. The temperature profile 302 has gradually lower temperatures going from the surface 104 to the innermost part 112 of the core. The midpoint temperature M′ is at least 50% of the surface temperature S′.

Referring to FIG. 3, data points corresponding to points on the temperature profile curve 302 are shown in the following Table 2.

TABLE 2 Temperature profile 302 as a function of distance from the surface 104. Distance from Temperature Percentile from Surface 104 (mm) (deg. Celsius) Surface 104 0 850 0 5 850 20 6.25 825 25 10 750 40 12.5 658.33 50 18.75 429.167 75 25 200 100 Table 2 shows data points for the temperature profile 302 as a function of distance from the surface 104 in millimeters. The third column further shows the percentile of depth along which each temperature and depth data point falls. Thus, for example, at 5 mm from the surface, the temperature profile 302 has a temperature of 850 degrees Celsius, and this is 20^(th) percentile away from the surface 104 toward the innermost part 112 of the core (which is disposed along the longitudinal axis L of the crankshaft 100, in this example).

As discussed above, the midpoint temperature M′ is at least 50% of the surface temperature S′. The midpoint 308 is located at the 50^(th) percentile of the depth, or halfway between the surface 104 and the innermost part 112 of the core. In this particular example, the midpoint temperature M′ is about 658 degrees C., while the surface temperature S′ is 850 degrees C. Therefore, in this example, the midpoint temperature M′ is greater than 70% of the surface temperature S; in this case, about 77%. It should be understood, however, that the temperature profile 302 may have some variation without falling beyond the spirit and scope of the present disclosure. In some cases, the midpoint temperature M′ may be in the range of 70% to 80% of the surface temperature S′. Similarly, the other temperatures in Table 2 may vary, for example, by up to 10%, or even up to 30%. For example, different materials could be used for the crankshaft 100 or other work piece, which would cause the temperature profile to be different than the exact temperature profile 302 shown in FIG. 3 and by Table 2.

As can be seen from the graph 300 and from Table 2, the temperature profile 302 includes a 25^(th) percentile temperature T₂₅′ at a 25^(th) percentile point 310 that is halfway between the midpoint 308 and the outer surface 104. In this case, the 25^(th) percentile temperature T₂₅′ is within 10% of the surface temperature S′. More particularly, in this case, the 25^(th) percentile temperature T₂₅′ is 825 degrees Celsius. The surface temperature S′ is 850 degrees Celsius, and therefore, the 25^(th) percentile temperature T₂₅′ is greater than 97% of the surface temperature S′, but less than 98%.

Furthermore, as can be seen from the graph 300 and from Table 2, the temperature profile 302 includes a 75^(th) percentile temperature T₇₅′ at a 75^(th) percentile point 312 that is halfway between the midpoint 308 and the innermost part 112 of the core. In this case, the 75^(th) percentile temperature T₇₅′ is at least 50% of the surface temperature S′.

Referring now to FIG. 4, a graph 400 illustrates a proportion of martensitic phase, after quenching, as a function of depth from the surface 104 of the crankshaft pin journal 108 to an innermost part 112 of the core. The curve 402 represents the martensitic phase data points, where depth in millimeters is illustrated on the horizontal axis 404 and percentage of martensite is illustrated along the vertical axis 406. FIG. 4 represents the martensite that is formed after first applying the temperature profile 302 shown in FIG. 3, and then quenching. 100% martensite was observed at the surface 104, while 0% martensite was observed at the innermost part 112 of the core. The graph 400 illustrates that the martensite gradually declines along the depth, rather than quickly vanishing near the surface 104, which results in the stresses being more evenly distributed, which leads to less potential for cracking under service loading.

Although the method described herein is applied to a crankpin 108 of a crankshaft 100, it should be understood that the method may be applied to any other work piece for which hardening is desired without cracking.

This description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. 

What is claimed is:
 1. A method for heat treating a crankshaft for a vehicle propulsion system, the crankshaft being formed of a crankshaft material and having an outer surface, the method comprising: heating at least a portion of the crankshaft to a temperature profile having a surface temperature at the outer surface, the temperature profile including gradually lower temperatures from the outer surface to a core of the crankshaft, the temperature profile including a midpoint temperature at a midpoint between the outer surface and an innermost part the core, the midpoint temperature being at least 50% of the surface temperature as measured in the Celsius scale, the surface temperature being within a transformation range of the crankshaft material; and subsequently quenching the outer surface of the crankshaft.
 2. The method of claim 1, wherein heating the portion of the crankshaft includes induction heating the portion of the crankshaft.
 3. The method of claim 2, wherein the temperature profile includes a 25th percentile temperature at a 25^(th) percentile point that is halfway between the midpoint and the outer surface, the 25^(th) percentile temperature being within 10% of the surface temperature as measured in the Celsius scale.
 4. The method of claim 3, wherein the temperature profile includes a 75^(th) percentile temperature at a 75^(th) percentile point that is halfway between the midpoint and the innermost part of the core, the 75^(th) percentile temperature being at least 50% of the surface temperature as measured in the Celsius scale, the midpoint temperature being at least 70% of the surface temperature as measured in the Celsius scale.
 5. The method of claim 4, wherein the midpoint temperature is in the range of 70% to 80% of the surface temperature as measured in the Celsius scale.
 6. The method of claim 5, wherein the 75^(th) percentile temperature is in the range of 60% to 70% of the surface temperature as measured in the Celsius scale.
 7. The method of claim 6, wherein the outer surface is a round bearing journal surface of the crankshaft and the temperature profile extends along a radius of the bearing journal surface to the innermost part of the core.
 8. The method of claim 7, further comprising providing the crankshaft material as a steel having an ideal critical diameter (DI) less than 1.70.
 9. The method of claim 8, further comprising providing the steel as a carbon steel having at least 0.3 wt % carbon.
 10. The method of claim 9, wherein heating the portion of the crankshaft includes applying a plurality of inductive field pulses to the crankshaft.
 11. The method of claim 10, wherein heating the portion of the crankshaft includes applying a first inductive pulse having an intensity in the range of 2.0 to 2.5 J/C*mm², applying a second inductive pulse having an intensity in the range of 2.0 to 2.5 J/C*mm², and applying a third inductive pulse having an intensity in the range of 2.0 to 2.5 J/C*mm², and pausing between the application of each of the first, second, and third inductive pulses.
 12. The method of claim 11, further comprising applying the first inductive pulse for a first period, applying the second inductive pulse for a second period, and applying the third inductive pulse for a third period, each of the first, second, and third periods being in the range of 8 to 12 seconds.
 13. The method of claim 12, wherein inductively heating the portion of the crankshaft comprises applying an alternating current to a coiled conductor.
 14. A method for forming a crankshaft for a vehicle propulsion system, the method comprising: forming the crankshaft of a crankshaft material; forming a round bearing journal surface about the crankshaft; applying a series of inductive heating pulses to the crankshaft until the crankshaft has a gradual temperature profile extending normally from the round bearing journal surface to a core of the crankshaft, the gradual temperature profile including a surface temperature at the round bearing journal surface, the surface temperature being within a transformation range of the crankshaft material, the gradual temperature profile including a midpoint temperature at a midpoint between the round bearing journal surface and an innermost part of the core, the midpoint temperature being at least 50% of the surface temperature as measured in the Celsius scale; and quenching the round bearing journal surface to a temperature below the transformation temperature to harden the round bearing journal surface.
 15. The method of claim 14, wherein the gradual temperature profile includes: a 25^(th) percentile temperature at a 25^(th) percentile point that is halfway between the midpoint and the round bearing journal surface, the 25^(th) percentile temperature being within 10% of the surface temperature as measured in the Celsius scale; and a 75^(th) percentile temperature at a 75^(th) percentile point that is halfway between the midpoint and the innermost part of the core, the 75^(th) percentile temperature being at least 50% of the surface temperature as measured in the Celsius scale, the midpoint temperature being at least 70% of the surface temperature as measured in the Celsius scale.
 16. The method of claim 15, wherein the midpoint temperature is in the range of 70% to 80% of the surface temperature as measured in the Celsius scale, and wherein the 75^(th) percentile temperature is in the range of 60% to 70% of the surface temperature as measured in the Celsius scale.
 17. The method of claim 16, further comprising providing the crankshaft material as a carbon steel having at least 0.3 wt % carbon and an ideal critical diameter (DI) less than 1.70.
 18. The method of claim 17, wherein applying the series of inductive heating pulses comprises: applying a first inductive pulse having an intensity in the range of 2.0 to 2.5 J/C*mm²; applying a second inductive pulse having an intensity in the range of 2.0 to 2.5 J/C*mm²; applying a third inductive pulse having an intensity in the range of 2.0 to 2.5 J/C*mm²; and pausing between the application of each of the first, second, and third inductive pulses.
 19. The method of claim 18, further comprising applying the first inductive pulse for a first period, applying the second inductive pulse for a second period, and applying the third inductive pulse for a third period, each of the first, second, and third periods being in the range of 8 to 12 seconds, and wherein pausing includes deactivating the induction for a period between 1 and 3 seconds between the application of each of the first, second, and third inductive pulses.
 20. A method of induction hardening a work piece, the method comprising: providing a work piece formed of a work piece material and having an outer surface and a center; applying a series of inductive heating pulses to the work piece until the workpiece has a gradual temperature profile extending normally from the outer surface to the center of the work piece, the gradual temperature profile including a surface temperature at the outer surface, the surface temperature being within a transformation range of the work piece material, the gradual temperature profile including a midpoint temperature at a midpoint between the outer surface and the center, the midpoint temperature being at least 50% of the surface temperature as measured in the Celsius scale; and quenching the outer surface to a temperature below the transformation temperature to harden the outer surface. 